Low phosphorus chabazites

ABSTRACT

A catalyst washcoat is provided having a molecular sieve with a CHA crystal structure; about 0.5 to about 5.0 mol % phosphorus; and SiO2 and Al2O3 in a mole ratio of about 5 to about 40. The washcoat includes one or more promoters or stabilizers, and may be applied to a monolith substrate to produce a catalytically active article.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of the U.S. patent application Ser.No. 14/991,272, filed Jan. 8, 2016, and allowed on Sep. 28, 2017, nowU.S. Pat. No. 9,878,316, which is a continuation of U.S. patentapplication Ser. No. 14/236,734, filed on Feb. 3, 2014, and issued onFeb. 9, 2016, now U.S. Pat. No. 9,254,481, which is the National Phaseof PCT International Application No. PCT/US2012/048313, filed Jul. 26,2012, and claims priority benefit of U.S. Provisional Patent ApplicationNo. 61/512,229, filed Jul. 27, 2011 the disclosures of each of which areincorporated herein by reference in their entireties for all purposes.

BACKGROUND A.) Field of Use

The present invention relates to catalysts, systems, and methods thatare useful for treating an exhaust gas which occurs from combustinghydrocarbon fuel—more particularly exhaust gas containing nitrogenoxides, such as an exhaust gas produced by diesel engines, gas turbines,or coal-fired power plants.

B.) Description of Related Art

Exhaust gas is emitted when fuels such as natural gas, gasoline, dieselfuel, fuel oil or coal is combusted and is discharged into theatmosphere through an exhaust pipe, flue gas stack or the like. Thelargest portions of most combustion exhaust gas contain relativelybenign nitrogen (N₂), water vapor (H₂O), and carbon dioxide (CO₂); butthe exhaust gas also contains in relatively small part noxious and/ortoxic substances, such as carbon monoxide (CO) from incompletecombustion, hydrocarbons (HC) from un-burnt fuel, nitrogen oxides(NO_(x)) from excessive combustion temperatures, and particulate matter(mostly soot). Of particular relevance to the present invention is anexhaust gas containing NOx, which includes nitric oxide (NO), nitrogendioxide (NO₂), and nitrous oxide (N₂O), that is derived from lean burnengines such as diesel engines for mobile applications.

Often, systems for treating diesel engine exhaust gas include one ormore catalyst compositions coated on or diffused into a substrate toconvert certain or all of the noxious and/or toxic exhaust componentsinto innocuous compounds. One such conversion method, commonly referredto as Selective Catalytic Reduction (SCR), involves the conversion ofNO_(x) in the presence of a catalyst and with the aid of a reducingagent into elemental nitrogen (N₂) and water. In an SCR process, agaseous reductant, typically anhydrous ammonia, aqueous ammonia, orurea, is added to an exhaust gas stream prior to contacting thecatalyst. The reductant is absorbed onto a catalyst and the NO_(x)reduction reaction takes place as the gases pass through or over thecatalyzed substrate. The chemical equation for a stoichiometric reactionusing either anhydrous or aqueous ammonia for an SCR process is:4NO+4NH₃+3O₂→4N₂+6H₂O2NO₂+4NH₃+3O₂→3N₂+6H₂ONO+NO₂+2NH₃→2N₂+3H₂O

Known SCR catalysts include zeolites or other molecular sieves disposedon a monolithic substrate. Molecular sieves are microporous crystallinesolids with well-defined structures and generally contain silicon,aluminum and oxygen in their framework and can also contain cationswithin their pores. A defining feature of molecular sieves is that theirframeworks are made up of interconnected networks of moleculartetrahedrals. Aluminosilicate molecular sieves, for example, arearranged as an open network of corner-sharing [AlO₄]- and[SiO₄]-tetrahedrals. In the case of a silica tetrahedral, a silicon atomis at the center of the tetrahedral while the four surrounding oxygenatoms reside at the tetrahedral's corners. Two or more tetrahedrals canthen be linked together at their corners to form various crystallinestructures.

A molecular sieve framework is defined in terms of the geometricarrangement of its primary tetrahedral atoms “T-atoms” (e.g., Al andSi). Each T-atom in the framework is connected to neighboring T-atomsthrough oxygen bridges and these or similar connections are repeated toform a crystalline structure. Since the framework per se is merely thearrangement of these coordinated atoms, specific framework types do notexpressly depend on composition, distribution of the T-atoms, celldimensions or symmetry. Instead, a particular framework is dictatedsolely by the geometric arrangement of T-atoms. (Codes for specificframework types are assigned to established structures that satisfy therules of the IZA Structure Commission.) However, materials of differingcompositions, but arranged according to the same framework, can possessvery different physical and/or chemical properties.

Crystalline structures can be formed by linking individual unit cells ofthe same or different frameworks together in a regular and/or repeatingmanner. These crystalline structures may contain linked cages, cavitiesor channels, which are of a size to allow small molecules to enter—e.g.the limiting pore sizes can be between 3 and 20 Å in diameter. The sizeand shape of these microporous structures are important to the catalyticactivity of the material because they exert a steric influence on thereactants, controlling the access of reactants and products.

Of particular interest to the present invention are small pore molecularsieves, such as those having a chabazite (CHA) framework. Two particularmaterials that have CHA frameworks, the aluminosilicate SSZ-13 and thesilicoaluminophosphate SAPO-34, are known to be useful in SCR processesfor converting NO_(x) to N₂ and O₂ and for other catalytic processes andeach has separate advantages.

In addition to their porosity, molecular sieves often have otherelements introduced as extra-framework constituents to improve theircatalytic performance. For example, U.S. Pat. No. 5,472,594 suggeststhat incorporating phosphorus into a ZSM-5 zeolite provides acomposition having unique properties as a catalytic agent. However, thephosphorus described in the '594 patent is not present as a crystallineframework constituent i.e., it has not been substituted for silicon oraluminum atoms. Likewise U.S. Pat. No. 7,662,737 describes ZSM-5 havingfree phosphate and/or phosphates bonded to extra-framework aluminum.Other examples of extra-framework constituents include metals, such ascopper or iron.

Thus, a need remains for improved hydrothermally stable small poremolecular sieves having a high degree of catalytic activity.

SUMMARY OF THE INVENTION

Applicants have discovered a new, low-phosphorus molecular sieve havinga CHA framework ([Al—Si—P—O]-CHA). The novel molecular sieve contains asmall amount of phosphorus in CHA framework comprising mostly silica andalumina. The presence of a small amount of phosphorus in the frameworkwas found to increase the molecular sieve's hydrothermal stability ofits Si/Al counterpart and provides similar or improved catalyticperformance. In addition, applicants have discovered that phosphorusT-atoms concentrated in discrete regions (i.e., clusters) of a crystalvis-à-vis evenly distributing the phosphorus atoms throughout a crystalimproves the performance of the material. Such materials have been foundto be particularly useful for reducing NOx in exhaust gas generated bydiesel engines.

Accordingly, provided is a composition comprising a crystallinestructure, wherein at least a portion of the crystalline structure is amolecular sieve having a CHA framework consisting of thirty-six T-atomsselected from the group consisting of silicon, aluminum, and phosphorus;wherein said molecular sieve comprises about 0.05 to about 5.0 molepercent of framework phosphorus based on the total moles of frameworksilicon, aluminum, and phosphorus in said molecular sieve; and whereinsaid molecular sieve has a silica-to-alumina mole ratio of at leastabout 10. Here, a crystalline structure per se contains a lowconcentration of phosphorus and thus the composition is not merely aphysical mixture or blend of an aluminosilicate and a conventionalsilicoaluminophosphate, such as SAPO-34. Preferably the phosphorus ispresent as PO₂. More preferably, the PO₂ is heterogeneously distributedin individual crystals of the CHA molecular sieve.

In another aspect of the invention, provided is a composition comprisinga molecular sieve material having a CHA framework, wherein the frameworkconsists of periodic building units having 36 interlinked T-atomsselected from the group consisting of aluminum, silicon, and phosphorus,and wherein said molecular sieve material has a mean phosphorusconcentration of about 0.5 to about 1.4 atoms per periodic buildingunit, preferably from about 1.1 to about 1.3.

In yet another aspect of the invention, provided is a molecular sievecomposition comprising —SiO₂, —AlO₂, and —PO₂ in a CHA framework and asilica-to-alumina ratio greater than about 10, preferably about 10 toabout 500, more preferably about 10 to about 50, and even morepreferably about 10 to about 32.

Other aspects of the invention include a catalytically active washcoatcomprising the abovementioned low-phosphorus molecular sieve; acatalytically active article, such as a wall flow or flow throughfilter, comprising a catalytically active washcoat; and an engineexhaust gas treatment system comprising the catalytically active articleand a source of ammonia.

Also provided is a method for reducing NOx in an exhaust gas comprisingcontacting the gas with a catalyst described herein for a time,temperature, and reducing environment sufficient to reduce the level ofNOx compounds in the gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is drawing of a unit cell having a CHA framework;

FIG. 1B is drawing of tetrahedral structure based on T-atoms and anexemplary interconnection via oxygen bridges;

FIG. 1C is a drawing of secondary building units;

FIG. 1D is a drawing of interconnected CHA unit cells forming acrystalline structure;

FIG. 2 is a graphical representation of NO_(x) conversion dataassociated with one embodiment of the invention;

FIG. 3 is a graphical representation of NH₃ conversion data associatedwith one embodiment of the invention; and

FIG. 4 is a graphical representation of N₂O generation data associatedwith one embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The present invention relates to a novel crystalline material having aCHA framework and a relatively small amount of framework phosphorus. Inits calcined and anhydrous form, the crystalline material of the presentinvention has a composition involving the molar relationship:(a)X₂O₃:(b)YO₂:(c)(P₂O₅)wherein X is a trivalent element, such as aluminum, boron, iron, indium,and/or gallium, preferably aluminum; Y is a tetravalent element, such assilicon, tin, titanium and/or germanium, preferably silicon; c is about0.005 to about 0.050 (0.05-5 mol. %), preferably about 0.015 to about0.050 (1.5-5 mol. %), more preferably about 0.020 to about 0.036 (2-3.6mol. %), and even more preferably about 0.026 to about 0.030 (2.6 to 3mol. %) or about 0.031 to about 0.035 (3.1 to 3.5 mol. %); a+b+c=1.00;and the ratio of b:a is at least about 5, preferably about 5 to about40. Unlike the chabazite SSZ-13, which has no phosphorus (U.S. Pat. No.4,544,538) and its analog SAPO-34 which preferably has a minimumphosphorus content of much greater than 25 mol. % (U.S. Pat. No.4,440,871), the novel molecular sieve of the present invention has aphosphorus content of about 0.5 to about 5 mole percent and preferablyan SAR of at least 5. The molecular sieve of the present inventionpossesses uniquely beneficial physical and chemical properties comparedto its closest molecular sieve analogs and thus represents a novelcomposition having a CHA framework.Composition:

Referring to FIG. 1A, shown is a unit cell of a CHA framework as definedby the International Zeolite Association (IZA). Each unit cell of theCHA framework has 36 tetrahedral subunits, wherein each subunit has acentral T-atom, preferably selected individually from Al, Si, and P.Preferably, individual crystals of the CHA crystalline material have amean phosphorus concentration of about 0.5 to about 2.0 per unit cell,more preferably from about 1.0 to about 1.5 per unit cell, and even morepreferably from about 1.1 to about 1.3 per unit cell. The position ofthe phosphorus atom(s) in the framework is not particularly limited.That is, the phosphorus atom(s) can be disposed at any position(s) inthe framework. Although each unit cell can contain one or morephosphorus atoms, the present invention is not so limited.Alternatively, the present invention includes crystalline materialshaving unit cells with no phosphorus atoms, provided that the meanphosphorus content of individual crystals is within the ranges describedabove.

The 36 tetrahedral subunits are connected via oxygen bridges (i.e.,co-sharing of oxygen atoms between two or more T-atoms) to formsecondary building units such as hexagons, stacked hexagons, squares, orbent rectangles, referred to in IZA nomenclature as (6), (6-6), (4), and(4-2), respectively (FIG. 1C). These secondary building units areconnected in the specific arrangement to produce a three-dimensionalunit cell of according to the defined CHA framework (FIG. 1A). Aplurality of unit cells are connected in a three-dimensional array toform a molecular sieve crystal (FIG. 1D).

Preferably, unit cells are based primarily on [AlO₄]- and[SiO₄]-moieties having a tetrahedral structure (FIG. 1B). Since most ofthese tetrahedral structures are connected to one another via oxygenbridges, most tetrahedrals will share oxygen atom corners. Accordingly,a majority of the unit cell comprises repeating —SiO₂ and/or —AlO₂moieties. A molecular sieve according to the present invention can beobtained with the desired silica-to-alumina ratio (SAR) (i.e., for thedesired catalytic application) by adjusting the synthesis parameters. Incertain embodiments, the molecular sieve has an SAR of at least about 5,more preferably from about 5 to about 150, more preferably from about 5to about 50, more preferably from about 5 to about 40. In certainpreferred embodiments, the molecular sieve has an SAR of about 5 toabout 32. In certain other preferred embodiments, the molecular sievehas an SAR of about 10 to about 32. As used herein, the term silicarefers to [SiO₄]- and/or —SiO₂ framework moieties, and the term aluminarefers to [AlO₄]—, —AlO₂, and/or Al₂O₃ framework moieties.

Interspersed in at least a portion of the unit cells are one or morephosphorus atoms. These phosphorus atoms preferably reside at T-atompositions and are preferably linked to aluminum atoms via oxygenbridges. Preferably the phosphorus is present as PO₂ moieties, morepreferably as PO₂ paired with one or more AlO₂ moieties. It is believedthat exchanging aluminum atoms of the molecular sieve with phosphorusatoms increases the hydrothermal stability and/or catalytic performanceof the molecular sieve. Thus, in certain preferred embodiments, themolecular sieve comprises at least about 80 mol. % SiO₂ and not morethan about 20 combined mol. % of said AlO₂ and said PO₂, preferablyabout 80 to about 90 mol % SiO₂ and about 10 to about 20 combined mol. %of said AlO₂ and said PO₂, wherein the combined mol. % of said AlO₂ andsaid PO₂ contains from about 0.5 to 5.0 mol. % PO₂, based on the totalcombined moles of silicon, aluminum, and phosphorus. In certainembodiments, the molecular sieve comprises about 85 to about 88 mol % ofsaid SiO₂, about 12 to about 15 combined mol. % of said AlO₂ and saidPO₂, wherein the combined mol. % of said AlO₂ and said PO₂ contains fromabout 0.5 to 5.0 mol. % PO₂, based on the total combined moles ofsilicon, aluminum, and phosphorus.

In certain embodiments, the molecular sieve composition comprises SiO₂,AlO₂, and PO₂ in a CHA framework; a silica-to-alumina ratio of about 12to about 32; and an amount of phosphorus in said framework to produce amean number of cation exchange site per periodic building unit of about0.5 to about 4.

In certain embodiments, molecular sieve crystals have a mean crystalsize of greater than 0.05 microns, preferably greater than 0.5 microns.Preferred molecular sieves have a mean crystal size of at least 1.0 μm.In certain embodiments, the molecular sieves have a mean crystal size ofabout 1.0 μm to about 5.0 μm, and more preferably about 1.5 μm to about2.5 μm, as calculated by methods described herein below. The accuratedirect measurement of the crystal size of molecular sieve materials isfrequently very difficult. Microscopy methods, such as SEM and TEM, maybe used. These methods typically require measurements of a large numberof crystals and, for each crystal measured, values may be evaluated inup to three dimensions. Furthermore, in order to more completelycharacterize the crystal size of a batch of crystals, one shouldcalculate the average crystal size, as well as the degree of variancefrom this average in terms of a crystal size distribution. For example,measurement by SEM involves examining the morphology of materials athigh magnifications (typically 1000× to 10,000×). The SEM method can beperformed by distributing a representative portion of the molecularsieve powder on a suitable mount such that individual particles arereasonably evenly spread out across the field of view at 1000× to10,000× magnification. From this population, a statistically significantsample (n) of random individual crystals (e.g., 200) are examined andthe longest dimension of the individual crystals parallel to thehorizontal line of the straight edge are measured and recorded.(Particles that are clearly large polycrystalline aggregates should notbe included the measurements.) Based on these measurements, thearithmetic mean and the variance of the sample are calculated. Incertain embodiments, the sample also has a mathematical variance aboutthe mean of less than 1, preferably less than 0.5, and even morepreferably less than 0.2.

In one embodiment, at least one non-aluminum base metal is used inconjunction with the molecular sieve to increase the catalyst'sperformance. As used herein, the phrase “molecular sieve catalystcontaining at least one a non-aluminum base metal” means a molecularsieve structure to which one or more base metals other than aluminum hasbeen added by ion exchange, impregnation, isomorphous substitution, etc.Moreover, the terms “base metal-containing molecular sieve catalyst” and“molecular sieve catalyst containing at least one base metal” are usedinterchangeably herein. As used herein, the term “base metal” means atleast one transition metal selected from copper, lead, nickel, zinc,iron, tin, tungsten, cerium, molybdenum, tantalum, magnesium, cobalt,bismuth, cadmium, titanium, zirconium, antimony, manganese, chromium,vanadium, ruthenium, rhodium, palladium, gold, silver, indium, platinum,iridium, rhenium, and niobium, and mixtures thereof. Preferred basemetals include those selected from the group consisting of chromium(Cr), cerium (Ce), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni),and copper (Cu), and mixtures thereof. Preferably, at least one of themetals is copper (Cu). Other preferred metals include iron (Fe) andcerium (Ce), particularly in combination with copper (Cu). In anotherpreferred embodiment, the base metal ions can be incorporated during thesynthesis steps of a molecular sieve, such as |Cu| [Al—Si—P—O]-CHA.

The amount of metal in the material can be adjusted depending on theparticular application and the particular metal. For example, molecularsieves of the present invention that are used as a mobile engine (e.g.,diesel) SCR catalyst in an ammonia-based reductant system preferablyinclude copper and/or iron. For embodiments utilizing copper, themolecular sieve contains about 0.5 to about 5 weight percent copper. Forhigh temperature stationary applications, such as boilers, molecularsieves according to the present invention may include 0.1 to about 1.5weight percent copper.

Preferably, the catalyst of the present invention is free or essentiallyfree of extra-framework phosphorus. Examples of extra-frameworkphosphorus include phosphorus on the surface or within the pores of themolecular sieve, and include phosphorus added to the molecular sievematerial by ion exchange, incipient wetness, spray drying, or otherknown techniques. By essentially free, it is meant that the catalystdoes not contain extra-framework phosphorus in an amount that affectsthe catalyst's SCR or AMOX performance. In certain embodiments, theamount of extra-framework phosphorus is less than about 1.5 weightpercent, preferably less than about 0.5 weight percent, more preferablyless than about 0.1 weight percent, and even more preferably less thanabout 0.01 weight percent, based on the total weight of the molecularsieve.

Synthesis:

Low phosphorus CHA molecular sieves of the present invention can besynthesized during formation of the molecular sieve structure (i.e., insitu) or can be incorporated into a molecular sieve structure after itis formed (i.e., post molecular sieve synthesis).

In one embodiment of a post molecular sieve synthesis method, alow-phosphorus CHA molecular sieve is synthesized by modifying astarting material having a CHA framework and essentially no phosphorus,such as the aluminosilicate, SSZ-13. That is, phosphorus is incorporatedinto a sample of SSZ-13 by treating the SSZ-13 with one or morephosphorus modifying compounds under conditions effective to exchange aportion of the framework aluminum atoms for phosphorus atoms. Forpurposes of illustration of a synthesis method, reference will be madeto SSZ-13 as a representative CHA molecular sieve. It is understood,however, that the starting materials of the present invention are notlimited to SSZ-13.

In one embodiment of an in-situ method, the amount of frameworkphosphorus can be controlled by adjusting the stoichiometric ratio ofthe reagents. For example, adjusting the concentrations of the Al₂O₃ andP₂O₅ reagents relative to the SiO₂ reagent during the formation ofmolecular sieve via a conventional templating synthesis method and canproduce a molecular sieve having a low amount of frame work phosphorusaccording to the present invention. In another example, theconcentration of a non-phosphorus component, such as metal oxide (e.g.,NaO) or other component to promote ion exchange during synthesis, can beadjusted during the formation of molecular sieve via a conventionaltemplating synthesis method to produce a molecular sieve having a lowamount of frame work phosphorus according to the present invention.

In another embodiment of an in-situ method, the order of reagentadditions can be modified to control the amount of framework phosphorusin a molecular sieve. For example, the addition of a phosphoruscontaining component into a templating mixture prior to the addition ofan alumina component can be used to modify the concentration offramework phosphorus.

In general, the number of cation exchange sites for such material isdependent upon the SAR and corresponding PO₂ molar ratio, as shown inTable 1.

TABLE 1 Aluminosilicate CHA with low-level Phosphorus - Unit CellComposition SiO2 AlO2 PO2 SAR Cation Exchange Sites per Unit Cell 32 3 121.3 2 32 2 2 32 0 31 4 1 15.5 3 31 3 2 20.7 1 30 5 1 12 4 30 4 2 15 230 3 3 20 0

Molecular sieves with application in the present invention can includethose that have been treated to improve hydrothermal stability.Conventional methods of improving hydrothermal stability include: (i)dealumination by steaming and acid extraction using an acid orcomplexing agent e.g. (EDTA—ethylenediaminetetracetic acid); treatmentwith acid and/or complexing agent; treatment with a gaseous stream ofSiCl₄ (replaces Al in the molecular sieve framework with Si); and (ii)cation exchange—use of multi-valent cations such as lanthanum (La).

Applications:

The molecular sieve catalyst for use in the present invention can be inthe form of a washcoat, preferably a washcoat that is suitable forcoating a substrate, such as a metal or ceramic flow through monolithsubstrate or a filtering substrate, including for example a wall-flowfilter or sintered metal or partial filter. Accordingly, another aspectof the invention is a washcoat comprising a catalyst component asdescribed herein. In addition the catalyst component, washcoatcompositions can further comprise a binder selected from the groupconsisting of alumina, silica, (non zeolite) silica-alumina, naturallyoccurring clays, TiO₂, CeO₂, ZrO₂, SnO₂, and mixtures of these.

In one embodiment, provided is a substrate upon which the molecularsieve catalyst is deposited. Preferred substrates for use in mobileapplication are monoliths having a so-called honeycomb geometry whichcomprises a plurality of adjacent, parallel channels, each channeltypically having a square cross-sectional area. The honeycomb shapeprovides a large catalytic surface with minimal overall size andpressure drop. The molecular sieve catalyst can be deposited on and/orin a flow-through monolith substrate (e.g., a honeycomb monolithiccatalyst support structure with many small, parallel channels runningaxially through the entire part) or filter monolith substrate such as awall-flow filter, etc. In another embodiment, the molecular sievecatalyst is deposited on and/or in a plate substrate for stationaryapplications, such as gas turbines and coal-fired power plants. Inanother embodiment, the molecular sieve catalyst is formed into anextruded-type catalyst. Preferably, the molecular sieve catalyst iscoated on a substrate in an amount sufficient to reduce the NOxcontained in an exhaust gas stream flowing through the substrate. Incertain embodiments, at least a portion of the substrate may alsocontain a platinum group metal, such as platinum (Pt), to oxidizeammonia in the exhaust gas stream.

The molecular sieves for use in the present invention also can besynthesized directly onto the substrate.

The molecular sieve catalysts according to the invention also can beformed into an extruded-type flow through catalyst.

The catalytic molecular sieves described herein can promote the reactionof a reductant, preferably ammonia, with nitrogen oxides to selectivelyform elemental nitrogen (N₂) and water (H₂O) vis-à-vis the competingreaction of oxygen and ammonia. In one embodiment, the catalyst can beformulated to favor the reduction of nitrogen oxides with ammonia (i.e.,and SCR catalyst). In another embodiment, the catalyst can be formulatedto favor the oxidation of ammonia with oxygen (i.e., an ammoniaoxidation (AMOX) catalyst). In yet another embodiment, an SCR catalystand an AMOX catalyst are used in series, wherein both catalysts comprisethe metal containing molecular sieve described herein, and wherein theSCR catalyst is upstream of the AMOX catalyst. In certain embodiments,the AMOX catalyst is disposed as a top layer on an oxidativeunder-layer, wherein the under-layer comprises a platinum group metal(PGM) catalyst or a non-PGM catalyst.

The reductant (also known as a reducing agent) for SCR processes broadlymeans any compound that promotes the reduction of NOx in an exhaust gas.Examples of reductants useful in the present invention include ammonia,hydrazine or any suitable ammonia precursor, such as urea ((NH₂)₂CO),ammonium carbonate, ammonium carbamate, ammonium hydrogen carbonate orammonium formate, and hydrocarbons such as diesel fuel, and the like.Particularly preferred reductants are nitrogen based, with ammonia beingparticularly preferred.

According to another aspect of the invention, provided is a method forthe reduction of NOx compounds or oxidation of NH₃ in a gas, whichcomprises contacting the gas with a catalyst composition describedherein for the catalytic reduction of NO_(x) compounds for a timesufficient to reduce the level of NO_(x) compounds in the gas. In oneembodiment, nitrogen oxides are reduced with the reducing agent at atemperature of at least 100° C. In another embodiment, the nitrogenoxides are reduced with the reducing agent at a temperature from about150 to 750° C. In a particular embodiment, the temperature range is from175 to 650° C. In another embodiment, the temperature range is from 175to 550° C. In yet another embodiment, the temperature range is 450 to750° C., preferably 450 to 700° C., even more preferably 450 to 650° C.Embodiments utilizing temperatures greater than 450° C. are particularlyuseful for treating exhaust gases from a heavy and light duty dieselengine that is equipped with an exhaust system comprising (optionallycatalyzed) diesel particulate filters which are regenerated actively,e.g. by injecting hydrocarbon into the exhaust system upstream of thefilter, wherein the molecular sieve catalyst for use in the presentinvention is located downstream of the filter.

In certain embodiments, the catalyst can be used in processes thatrequire very low N₂O production. Specific applications include systemsand methods for treating lean-burn exhaust gases at low temperaturessuch as below 200° C., below about 250° C., about 150 to about 300° C.,or about 200° C. to about 400° C. Such systems include engines that haveone or more major exhaust gas cycles within one of these temperatureranges, or exhaust systems or engines that are designed to treat exhaustgas within one of these ranges for a significant portion (e.g., at least25%) or even a majority of their operational time. Other applicationsinclude engines or other combustion processes that tuned in a mannerthat produces large amounts of NO₂. In certain embodiments, themolecular sieve having a low amount of framework phosphorus, includingmethods and systems utilizing the same, produce less N₂O compared to asimilar catalyst material having no frame-work phosphorus, such asaluminosilicates. In certain embodiments, the N₂O production is lessthan 5 ppm, more preferably less than about 1 ppm, and even morepreferably less than about 0.1 ppm per 500 ppm NO being treated at atemperature less than about 400° C., more preferably from about 175° C.to about 375° C., such as about 200 to about 350° C. or about 200 toabout 300° C.

In another embodiment, the nitrogen oxides reduction is carried out inthe presence of oxygen. In an alternative embodiment, the nitrogenoxides reduction is carried out in the absence of oxygen.

The method can be performed on a gas derived from a combustion process,such as from an internal combustion engine (whether mobile orstationary), a gas turbine and coal or oil fired power plants. Themethod may also be used to treat gas from industrial processes such asrefining, from refinery heaters and boilers, furnaces, the chemicalprocessing industry, coke ovens, municipal waste plants andincinerators, etc. In a particular embodiment, the method is used fortreating exhaust gas from a vehicular lean burn internal combustionengine, such as a diesel engine, a lean-burn gasoline engine or anengine powered by liquid petroleum gas or natural gas.

According to a further aspect, the invention provides an exhaust systemfor a vehicular lean burn internal combustion engine, which systemcomprising a conduit for carrying a flowing exhaust gas, a source ofnitrogenous reductant, and a molecular sieve catalyst described herein.The system can include means, when in use, for controlling the meteringmeans so that nitrogenous reductant is metered into the flowing exhaustgas only when it is determined that the molecular sieve catalyst iscapable of catalyzing NO_(x) reduction at or above a desired efficiency,such as at above 100° C., above 150° C. or above 175° C. Thedetermination by the control means can be assisted by one or moresuitable sensor inputs indicative of a condition of the engine selectedfrom the group consisting of: exhaust gas temperature, catalyst bedtemperature, accelerator position, mass flow of exhaust gas in thesystem, manifold vacuum, ignition timing, engine speed, lambda value ofthe exhaust gas, the quantity of fuel injected in the engine, theposition of the exhaust gas recirculation (EGR) valve and thereby theamount of EGR and boost pressure.

In a particular embodiment, metering is controlled in response to thequantity of nitrogen oxides in the exhaust gas determined eitherdirectly (using a suitable NOx sensor) or indirectly, such as usingpre-correlated look-up tables or maps—stored in the controlmeans—correlating any one or more of the abovementioned inputsindicative of a condition of the engine with predicted NO_(x) content ofthe exhaust gas. The metering of the nitrogenous reductant can bearranged such that 60% to 200% of theoretical ammonia is present inexhaust gas entering the SCR catalyst calculated at 1:1 NH₃/NO and 4:3NH₃/NO₂. The control means can comprise a pre-programmed processor suchas an electronic control unit (ECU).

In a further embodiment, an oxidation catalyst for oxidizing nitrogenmonoxide in the exhaust gas to nitrogen dioxide can be located upstreamof a point of metering the nitrogenous reductant into the exhaust gas.In one embodiment, the oxidation catalyst is adapted to yield a gasstream entering the SCR molecular sieve catalyst having a ratio of NO toNO₂ of from about 4:1 to about 1:3 by volume, e.g. at an exhaust gastemperature at oxidation catalyst inlet of 250 to 450° C. The oxidationcatalyst can include at least one platinum group metal (or somecombination of these), such as platinum, palladium, or rhodium, coatedon a flow-through monolith substrate. In one embodiment, the at leastone platinum group metal is platinum, palladium or a combination of bothplatinum and palladium. The platinum group metal can be supported on ahigh surface area washcoat component such as alumina, a molecular sievesuch as an aluminosilicate molecular sieve, silica, non-zeolite silicaalumina, ceria, zirconia, titania or a mixed or composite oxidecontaining both ceria and zirconia.

In a further embodiment, a suitable filter substrate is located betweenthe oxidation catalyst and the molecular sieve catalyst. Filtersubstrates can be selected from any of those mentioned above, e.g. wallflow filters. Where the filter is catalyzed, e.g. with an oxidationcatalyst of the kind discussed above, preferably the point of meteringnitrogenous reductant is located between the filter and the molecularsieve catalyst. Alternatively, if the filter is uncatalyzed, the meansfor metering nitrogenous reductant can be located between the oxidationcatalyst and the filter.

In a further embodiment, the molecular sieve catalyst for use in thepresent invention is coated on a filter located downstream of theoxidation catalyst. Where the filter includes the molecular sievecatalyst for use in the present invention, the point of metering thenitrogenous reductant is preferably located between the oxidationcatalyst and the filter.

In a further aspect, there is provided a vehicular lean-burn enginecomprising an exhaust system according to the present invention. Thevehicular lean burn internal combustion engine can be a diesel engine, alean-burn gasoline engine or an engine powered by liquid petroleum gasor natural gas.

EXAMPLES

To better understand the invention, the following non-limiting examplesare provided for illustrative purposes.

Example 1—In-Situ Synthesis of a Low-Phosphorus CHA Molecular Sieve

A sol-gel reaction composition was prepared by combining 75.80 g fumedsilica with 500 g de-mineralized water with stirring until homogeneous.Approximately 5.27-5.89 g sodium aluminate was dissolved in 209.11 g ofN,N,N-trimethyl-1-adamantammonium hydroxide, TMADOH (25.5 wt. %)followed by the dissolution of 6.75-7.10 g sodium hydroxide. To the basesolution 0.364-1.091 g orthophosphoric acid (85 wt. %) was homogenized.Still under stirring the silica source was added to the base solutionfollowed by 240 g de-mineralized water and the mixture left to stiruntil homogeneous. The pH was measured before charging to a stainlesssteel pressure reactor. Finally, under stirring, de-min water was addedto the reactor until the sol-gel reached the following molarcomposition:60SiO₂:1.275≤x≤1.425Al₂O₃:0.075≤y≤0.225P₂O₅:6Na₂O:12TMADOH:2640H₂OThe reactor was purged with nitrogen and the mixture crystallized at170° C. after 3 days.

This procedure was repeated using the relative amounts of reagents shownin Table 2.

TABLE 2 In-situ Synthesis (components expressed in mol. %) Tri-MeExample SiO2 Al2O3 Fe2O3 P2O5 Na2O NaNO3 ADAM OH H2O 1a 60 1.425 0 0.0756 0 12 2640 1b 60 1.3875 0 0.1125 6 0 12 2640 1c 60 1.35 0 0.15 6 0 122640 1d 60 1.3125 0 0.1875 6 0 12 2640 1e 60 1.275 0 0.225 6 0 12 2640These experiments produce a molecular sieve having a CHA framework and avery low amount of framework phosphorus.

The materials were calcined increasing the temperature of the materialfrom room temperature to 110° C. at a rate of 2° C./minute and underdrying conditions, then increasing the temperature of the material to450° C. at a rate of 5° C./minute, holding the material at 450° C. for16 hours, the further increasing the temperature of the material to 550°C. at a rate of 5° C./minute and holding the material at 550° C. foranother 16 hours. The material was then allowed to cool to roomtemperature again.

The cooled material was loaded with copper by weight via an incipientwetness process.

The ion-exchanged material was then activated by increasing thetemperature of the material from room temperature to 150° C. at a rateof 2° C./minute, holding the material at 150° C. for 16 hours, thenincreasing the temperature of the material to 450° C. at a rate of 5°C./minute, holding the material at 450° C. for 16 hours. The materialwas then allowed to cool to room temperature again.

Example 2—In-situ Synthesis of a Low-Phosphorus CHA Molecular Sieve

A sol-gel reaction composition was prepared by combining 75.80 g fumedsilica with 500 g de-mineralized water with stirring until homogeneous.Approximately 5.27 g sodium aluminate was dissolved in 209.11 g ofN,N,N-trimethyl-1-adamantammonium hydroxide, TMADOH (25.5 wt. %)followed by the dissolution of 0.37-7.10 g sodium hydroxide. To the basesolution 1.091 g orthophosphoric acid (85 wt. %) was homogenised. Stillunder stirring the silica source was added to the base solution followedby 240 g de-mineralized water and the mixture left to stir untilhomogeneous. The pH was measured before charging to a stainless steelpressure reactor. Finally, under stirring, de-min water was added to thereactor until the sol-gel reached the following molar composition:60SiO₂:1.275Al₂O₃:0.225P₂O₅:2≤x≤6Na₂O:12TMADOH:2640H₂OThe reactor was purged with nitrogen and the mixture crystallized at170° C. after 3 days.

This procedure was repeated using the relative amounts of reagents shownin Table 3.

TABLE 3 In-situ Synthesis (components expressed in mol. %) Tri-MeExample SiO2 Al2O3 Fe2O3 P2O5 Na2O NaNO3 ADAM OH H2O 2a 60 1.275 0 0.2256 0 12 2640 2b 60 1.275 0 0.225 5 0 12 2640 2c 60 1.275 0 0.225 4 0 122640 2d 60 1.275 0 0.225 3 0 12 2640 2e 60 1.275 0 0.225 2 0 12 2640These experiments produce a molecular sieve having a CHA framework and avery low amount of framework phosphorus.

The materials were calcined, ion-exchanged with copper, and thenactivated using a process similar to that described in Example 1.

Example 3—In-Situ Synthesis of a Low-Phosphorus CHA Molecular Sieve

-   A sol-gel reaction composition was prepared by two methods:

(a) combining 75.80 g fumed silica with 500 g de-min water with stirringuntil homogeneous. 5.27 g sodium aluminate was dissolved in 209.11 g ofN,N,N-trimethyl-1-adamantammonium hydroxide, TMADOH (25.5 wt. %)followed by the dissolution of 7.10 g sodium hydroxide. To the basesolution 1.091 g orthophosphoric acid (85 wt. %) was homogenised. Stillunder stirring the silica source was added to the base solution followedby 240 g de-min water and the mixture left to stir until homogeneous.The pH was measured before charging to a stainless steel pressurereactor. Finally, under stirring, de-min water was added to the reactoruntil the sol-gel reached the following molar composition:60SiO₂:1.275Al₂O₃:0.225P₂O₅:6Na₂O:12TMADOH:2640H₂OThe reactor was purged with nitrogen and the mixture crystallized at170° C. after 3 days; and

(b) combining 75.80 g fumed silica with 500 g de-min water with stirringuntil homogeneous followed by the addition of 1.091 g orthophosphoricacid (85 wt. %). 5.27 g sodium aluminate was dissolved in 209.11 g ofN,N,N-trimethyl-1-adamantammonium hydroxide, TMADOH (25.5 wt. %)followed by the dissolution of 7.10 g sodium hydroxide. Still understirring the silica source was added to the base solution followed by240 g de-min water and the mixture left to stir until homogeneous. ThepH was measured before charging to a stainless steel pressure reactor.Finally, under stirring, de-min water was added to the reactor until thesol-gel reached the following molar composition:60SiO₂:1.275Al₂O₃:0.225P₂O₅:6Na₂O:12TMADOH:2640H₂OThe reactor was purged with nitrogen and the mixture crystallized at170° C. after 3 days.

The materials were calcined, ion-exchanged with copper, and thenactivated using a process similar to that described in Example 1

Example 3—SCR Activity

The fresh and aged |Cu| [Al—Si—P—O]-CHA material was tested using aSynthetic Catalyst Activity Test (SCAT) rig under the followingconditions: 500 ppm NO, 500 ppm NH₃, 10% O₂, 10% H₂O and the balance N₂;and a space velocity (SV) of 60,000/hour. For comparison, similartesting was performed on a sample of copper loaded SSZ-13.

The samples were tested to determine NO_(x) conversion, NH₃ conversion,and N₂O production, each as a function of temperature. NO_(x) conversiondata is shown in FIG. 2, NH₃ conversion data is shown in FIG. 3, and N₂Oproduction data is shown in FIG. 4.

The results show that the |Cu| [Al—Si—P—O]-CHA material according to thepresent invention and SSZ-13 have comparable performance.

Example 4—Synthesis of a Low-Phosphorus CHA Molecular Sieve

Additional synthesis of low-phosphorus CHA molecular sieve are conductedto produce materials having about 0.05 mol % phosphorus, 0.10 mol %phosphorus, 0.25 mol % phosphorus, 0.50 mol % phosphorus, and 1.0 mol %phosphorus.

What is claimed is:
 1. A method for reducing NOx in an exhaust gascomprising contacting the gas with a catalyst for a time and temperaturesufficient to reduce the level of NOx compounds in the gas, wherein thecatalyst is a catalyst comprising a composition comprising a molecularsieve material having a CHA framework, wherein the framework consists ofperiodic building units having 36 interlinked T-atoms selected from thegroup consisting of aluminum, silicon, and phosphorous, and wherein saidmolecular sieve material has a mean phosphorous concentration of about0.5 to about 1.5 atoms per periodic building unit.
 2. The method ofclaim 1, wherein said mean phosphorus concentration is about 1.0 toabout 1.5 atoms per periodic building unit.
 3. The method of claim 1,wherein said molecular sieve material has a silica-to-alumina ratio ofat least about
 10. 4. The method of claim 1, wherein said molecularsieve material has a silica-to-alumina ratio of at least about 10 and anAl:P ratio of greater than about
 1. 5. The method of claim 1, whereinsaid molecular sieve material has a silica-to-alumina ratio of at leastabout 10 and an Al:P ratio of greater than about
 10. 6. The method ofclaim 1, wherein said molecular sieve material has a silica-to-aluminaratio of at least about 10 and an Al:P ratio of greater than about 100.7. The method of claim 1, wherein said phosphorus is connected to atleast a portion of said aluminum via oxygen bridges.
 8. The method ofclaim 1, wherein said molecular sieve material further comprises anon-aluminum base metal.
 9. The method of claim 8, wherein saidnon-aluminum base metal is ion-exchanged.
 10. The method of claim 9,wherein said non-aluminum base metal is copper.
 11. The method of claim9 wherein said non-aluminum base metal is iron.
 12. An engine exhaustgas treatment system comprising: a. a catalyst article comprising acatalytically active washcoat on a monolith substrate, wherein thecatalytically active washcoat comprises i. a molecular sieve materialhaving:
 1. a CHA framework;
 2. about 0.5 to about 5.0 mol % phosphorousbased on the total moles of framework silicon, aluminum, and phosphorousin said molecular sieve;
 3. SiO₂ and Al₂O₃ in a mol ratio of at leastabout 10;
 4. an aluminum to phosphorous ratio of at least about 1; and5. about 0.025 to about 5 w/w % of copper and/or iron on an anhydrousmolecular sieve basis; and ii. one or more promoters or stabilizers; andb. a source of ammonia or urea upstream of said catalyst article. 13.The engine exhaust gas treatment system of claim 12, wherein themonolith substrate comprises a wall flow filter substrate.
 14. Theengine exhaust gas treatment system of claim 12, wherein the monolithsubstrate comprises a flow-through substrate.
 15. The engine exhaust gastreatment system of claim 12, wherein said molecular sieve comprisesabout 0.05 to about 1.0 mole percent of framework phosphorus based onthe total moles of framework silicon, aluminum, and phosphorus in saidmolecular sieve.
 16. The engine exhaust gas treatment system of claim12, wherein said molecular sieve material has a silica-to-alumina ratioof at least about 10 and an Al:P ratio of greater than about
 1. 17. Theengine exhaust gas treatment system of claim 12, wherein said molecularsieve material has a silica-to-alumina ratio of at least about 10 and anAl:P ratio of greater than about
 10. 18. The engine exhaust gastreatment system of claim 12, wherein said molecular sieve material hasa silica-to-alumina ratio of at least about 10 and an Al:P ratio ofgreater than about
 100. 19. The engine exhaust gas treatment system ofclaim 12, wherein said copper or iron is ion exchanged.
 20. The engineexhaust gas treatment system of claim 12, wherein said molecular sievematerial contains ion exchanged copper.